Theoretical calculations of spin-Hamiltonian parameters are very useful not only in determination of structure but also in the identification of radicals in biological systems. In particular, we are most interested in predicting values of the g-tensor, since the g-tensor is the primary piece of information obtained from high-frequency EPR. With detailed information on variation of the g-tensor with structure of the radical, the structure can be elucidated, different radicals can be distinguished, and solvent effects gauged. With semi-empirical MO methods (AM1 and PM3), we can make very good approximations of the hyperfine and g-tensors. These calculations are primarily limited by how well the basis set determines the spin-density distribution. In most cases the spin-density is determined fairly accurately by such semi-empirical methods, but there are also many exceptions, particularly sigma radicals and certain non-planar pi radicals. More definitive results can be obtained from ab ini tio calculations. Because of the large number of C. I. states generated for extended basis sets, we are currently limited to small molecules. Using perturbation methods, we plan to extend our g-tensor calculations to large molecules. This is an ongoing project.